Heretofore, various liquid crystal display systems have been proposed. Examples of these systems include the following (refer to "The Latest Technology of Liquid Crystals," edited by Kogyo Chosa-kai (1983)). Systems which use liquid crystal compositions having positive dielectric anisotropy values include TN and STN systems, or active matrix (AM) (TFT or metal-insulation film-metal (MIM)) systems based on the TN mode. Systems which use liquid crystal compositions having negative dielectric anisotropy values include the ECB (HAN or DAP), DS, GH, and PC systems.
Of these systems, the system using liquid crystal compositions having positive dielectric anisotropy values constitutes the mainstream of practical use. In comparison, practical use of the system using liquid crystal compositions having negative dielectric anisotropy values has been delayed. In the light of the above, the development of liquid crystal compositions having negative dielectric anisotropy values and compounds used for producing such compositions is insufficient relative to the development of liquid crystal compositions having positive dielectric anisotropy values.
Under this situation, recent efforts have been made to improve the viewing angle, which is one of the shortcomings of liquid crystal displays. One system which addresses this fault is IPS (R. Kiefer et al. , JAPAN DISPLAY '92, 547 (1992) , M. Oh-e et al., ASIA DISPLAY '95, 577 (1995), Japanese Patent Application Laid-open No.5-505247, Japanese Patent Application Laid-open No. 7-128647, etc.). One of the features of IPS used in the liquid crystal panel of the cited invention is that a comb-shaped electrode is provided on only one side of substrates, whereas in conventional liquid crystal panels an electrode is provided on each of upper and lower substrates. Another feature of the cited invention is that the liquid crystal composition can be used regardless of whether the dielectric anisotropy value is positive or negative.
Another fruit of the efforts to improve the viewing angle is a system utilizingthe vertical orientation of liquid crystal molecules (Japanese Patent Application Laid-open No. 2-176625). One of the features of this system is the use of liquid crystal compositions having negative dielectric anisotropy values.
In view of such a background, liquid crystalline compounds and liquid crystal compositions having negative dielectric anisotropy values have been strongly desired.
In liquid crystal compositions used in display systems, not only dielectric anisotropy values, but also other properties, such as optical anisotropy values (.DELTA.n) and elastic coefficient ratio K.sub.33 /K.sub.11 (K.sub.33 : bend elastic coefficient, K.sub.11 : spray elastic coefficient), should be adjusted to optimal values, and furthermore, the liquid crystal phase must exist in an adequate temperature range, and the composition must have a low viscosity even at low temperature.
Among conventionally known liquid crystalline compounds, none satisfy all of these requirements. Therefore, compositions which can be used as a liquid crystal are obtained by mixing several to more than 20 kinds of species of compounds having liquid crystal phases, and, when desired, several types of compounds having no liquid crystal phases. Therefore, each liquid crystalline compound to be mixed is required properties such as high miscibility with other liquid crystalline compounds and high miscibility at a low temperature region for use at low temperature.
However, since as described above the mainstream of conventional display systems has involved the use of liquid crystal compositions containing liquid crystalline compounds having positive dielectric anisotropy values, the development of liquid crystal compounds or compositions having negative dielectric anisotropy values has not been sufficient. Therefore, conventional liquid crystalline compounds and compositions have been unable to support such diversified systems and satisfy accompanying requirements for various properties. For example, in a conventional liquid crystal composition,
even if .DELTA..epsilon. is negative, its absolute value is small; PA1 since the elastic coefficient is high, the driving voltage cannot be lowered; PA1 since miscibility is poor, such a compound cannot be used in a large quantity; PA1 the optical anisotropy values cannot be set freely; PA1 the viscosity is high; and PA1 chemical and physical stability is poor.
Prior to the present invention, compounds having 2,2-difluorocyclohexane-1,4-diyl, 6-fluorocyclohexene-1,4-diyl, or 2-fluorocyclohexene-1,4-diyl skeletons have been reported (Japanese Patent Application Laid-open Nos. 5-279279 and 8-12604, and German Patent Publication No. 4427266A1). However, these patents do not describe any of the properties that such compounds required for use for preparing liquid crystal compositions; for example, adequate .DELTA..epsilon. and .DELTA.n, low elastic coefficients, high miscibility, low viscosity, and high chemical and physical stability.
Also, these patents do not express the idea of converting cyclohexane skeletons to pyran skeletons in order to derive liquid crystal compounds having negative .DELTA..epsilon. from the compounds having 2,2-difluorocyclohexane-1,4-diyl, 6-fluorocyclohexene-1,4-diyl, or 2-fluorocyclohexene-1,4-diyl skeletons. Thus, the compound of the present invention cannot be derived easily from prior art.
The object of the present invention is to solve the problems described above, and to provide a novel liquid crystalline compound, a liquid crystal composition containing the liquid crystalline compound,, and a liquid crystal display element fabricated utilizing the liquid crystal composition, which can be used in a vertical orientation system as described in Japanese Patent Application Laid-open No.2-176625 and in various display systems using compounds or compositions having negative .DELTA..epsilon. values, such as IPS, ECB (HAN or DAP), DS, GH, or PC, as well as for the adjustment of properties of liquid crystal compositions for various display systems using compounds or compositions having positive .DELTA..epsilon. values, such as TN, STN, or AM (TFT or MIM) based on the TN mode.